The present invention relates generally to liquid cargo tank transport vehicles. More particularly, the present invention relates to an apparatus and method for removing liquid from the loading lines of the cargo tank, after loading or unloading, in order to prevent leakage or spillage of the liquid if the loading lines should become damaged during transportation.
Hazardous or volatile liquids such as gasoline or diesel fuel are typically transported in bottom loading cargo tanks. Normally, each cargo tank has four or five compartments with an external loading/unloading line (hereinafter xe2x80x9cwet linexe2x80x9d) mounted at the bottom center of each compartment. The cargo tank is loaded with liquid cargo which passes through the wet lines and into the compartments. After each compartment of the cargo tank is filled, a residual amount of liquid (perhaps 5-10 gallons) may remain in the associated wet line.
For safety reasons, it is desirable to not allow the volatile liquid to remain in the wet line during movement of the cargo tank from one site to the next. One method of removing the remaining liquid from the wet line is disclosed in U.S. Pat. No. 5,377,715 to Andenmatten, et al., which is incorporated by reference herein for this background explanation. The Andenmatten patent discloses a method of introducing compressed gas into the wet line in order to force the remaining liquid back into the cargo container via a fluid return line. However, if the compressed gas contains oxygen, it may mix with volatile vapors in the wet line to create a potentially explosive, pressurized vapor/oxygen combination. Even if an inert or non-oxygenated gas is pumped into the wet line, it still must remain in the wet line under pressure, putting stress on seals and posing the danger of unwanted escape into the environment. If the non-oxygenated gas is highly saturated vapor from the top of the cargo tank, the safety and environmental concerns regarding scaping gas are even greater. What is needed in the art is a method of returning the liquid to the cargo tank without pressurizing the wet line.
The present invention also includes an improved light tube optical sensor for determining when liquid is present in the wet lines. Existing light tube optical sensors such as U.S. Pat. No. 3,995,169 to Oddon have several shortcomings which hinder their use in environments such as wet lines. The Oddon optical sensor is a U-shaped light tube which receives light from a source at one end and under the proper circumstances, directs the light to a detector at the opposite end. When the refractive index between the light tube material (say 1.5 for glass) and the surrounding environment (say 1.0 for air) is significant, light tends to travel around the bend of the light tube and reaches the detector. Thus, when the bend of the light tube is surrounded by air, the detector can sense light. However, when the bend in the light tube becomes surrounded by a liquid having a higher refractive index (say 1.4 for gasoline), light largely exits the light tube and no longer reaches the detector. In this manner, it can be determined if a liquid has reached the level of the bend in the light tube.
The Oddon optical sensor has a light tube with flat surfaces at its bend. While this flat surface is intended to more efficiently direct light around the bend, it also is more likely to allow ambient light from outside the tube to enter and travel through the tube and be falsely interpreted by the detector. Additionally, Oddon uses a round, conventional light bulb spaced above several light tubes in order to inject light into all of these tubes. This is significant power wastage because light energy is propagated in all directions instead of being narrowly directed down the tubes. Moreover, Oddon is limited to determining whether or not the detector receives a certain amount of light energy. Oddon is not able to distinguish between a true signal (i.e. light coming directly from the light source) and a false signal (e.g. light exiting the tube, reflecting off a container wall, and re-entering the light tube). There is a need in the art for an optical sensor which overcomes the limitations found in prior art devices such as the Oddon sensor.
It is an object of the present invention to provide a system for returning fluid in a wet line to the main cargo container without the necessity of pressurizing the wet line.
It is the further object of the present invention to provide a system with an improved optical level sensor.
Therefore the present invention provides a system for returning residual liquid remaining in a loading line to a liquid cargo container after loading or unloading of the cargo container. This system includes a liquid return line extending between the loading line and the cargo container. A pump is positioned to move liquid from the loading line, through the liquid return line, and into the cargo container. A vapor line communicates between a vapor space in the cargo container and the loading line.
The present invention further comprises an optical liquid level sensor. The level sensor includes a light tube having two substantially straight sections joined by a substantially continuous curvature bend. The bend has a rounded cross-section and the light pipe is formed of a light conducting material. An optical emitter is positioned at the end of one of the straight sections of the pipe and an optical detector is positioned at the other straight section of the pipe. A micro-controller activates the optical emitter and monitors the optical sensor.